BMR explained: what it is and how to increase it

You've probably heard people say "I have a slow metabolism" when explaining why they struggle to lose weight. What they're usually referring to — without knowing the term — is their Basal Metabolic Rate, or BMR.

Understanding your BMR is one of the most useful things you can do for your long-term health and body composition goals.

What is BMR?

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — the minimum energy needed to keep your vital organs functioning. This includes your heart beating, lungs breathing, brain running, kidneys filtering, and cells regenerating.

BMR represents roughly 60–70% of your total daily calorie burn, making it by far the largest contributor to your TDEE. Even on a day you do no exercise at all, your body is burning hundreds of calories just to stay alive.

How is BMR calculated?

The most accurate widely-used formula is the Mifflin-St Jeor equation, published in 1990 and validated across multiple studies:

• Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

This formula accounts for the four main factors that determine BMR: body weight, height, age, and sex.

What affects your BMR?

1. Muscle mass

Muscle tissue is metabolically active — it burns calories even at rest. Fat tissue burns far fewer. This is why two people of the same weight can have very different BMRs if one has significantly more muscle. Building muscle is the most effective long-term strategy for raising your BMR.

2. Age

BMR declines with age, primarily because people tend to lose muscle mass as they get older (a process called sarcopenia). This is why it often becomes harder to maintain weight as you age — but it's not inevitable. Regular resistance training significantly slows this decline.

3. Body size

Larger bodies have more cells to maintain, so they burn more calories at rest. A taller, heavier person will generally have a higher BMR than a shorter, lighter one of the same age and sex.

4. Sex

Men typically have higher BMRs than women of the same size and age, largely due to having more muscle mass and less body fat on average.

5. Hormones

Thyroid hormones play a major role in regulating metabolic rate. An underactive thyroid (hypothyroidism) can meaningfully reduce BMR. If you suspect a hormonal issue affecting your metabolism, a doctor can test your thyroid function.

6. Genetics

There's a genuine genetic component to BMR, but its effect is smaller than most people assume. Lifestyle factors — particularly muscle mass and activity level — have a far greater impact over time.

Science-backed ways to increase your BMR

Build muscle through resistance training

This is the most impactful thing you can do. Each kilogram of muscle burns roughly 13 kcal per day at rest, compared to about 4.5 kcal for fat. Adding even 2–3 kg of muscle over several months can meaningfully raise your resting calorie burn. Aim for 2–4 resistance training sessions per week.

Eat enough protein

Protein has the highest thermic effect of any macronutrient — your body burns 20–30% of protein calories just digesting it, versus 5–10% for carbs and 0–3% for fat. High protein intake also helps preserve muscle during a calorie deficit.

Don't drop calories too low

Severe calorie restriction causes your body to downregulate your metabolism — a survival response called metabolic adaptation. Your BMR can drop by 10–15% on very low calorie diets. This is one reason crash diets don't work long-term: you end up burning fewer calories even at rest.

Stay consistently active

While individual exercise sessions don't change BMR directly, staying active long-term builds muscle and maintains metabolic rate. Consistent movement — even non-exercise activity like walking and standing — adds up significantly.

Sleep well

Chronic sleep deprivation disrupts hormones including leptin, ghrelin, and cortisol, which can negatively affect metabolism, hunger regulation, and body composition over time. Prioritising 7–9 hours of sleep is a genuine metabolic strategy.

BMR vs RMR — what's the difference?

You may also see the term RMR (Resting Metabolic Rate). It's similar to BMR but measured under slightly less strict conditions (you don't need to fast or sleep beforehand). RMR is typically 10–20% higher than true BMR. Most online calculators actually estimate RMR, though they label it as BMR — which is close enough for practical purposes.

Key takeaways

• BMR is the calories your body burns at complete rest — around 60–70% of your total daily burn.
• The main factors are muscle mass, age, body size, sex, and hormones.
• Building muscle through resistance training is the most effective way to raise your BMR long-term.
• Eating enough protein and avoiding extreme deficits helps protect your metabolic rate.
• BMR naturally declines with age, but regular training significantly slows this.